A quick overview about garbage collection algorithm in Java. CMS and G1 are explained and compared.

1. Garbage Collection in Java
“Heap storage for objects is reclaimed by an automatic
storage management system. Objects are never
explicitly de-allocated”
–Java Virtual Machine Specification [1]

2. Java Variable Flavours
• Stack
– Local variables holding
primitives.
10
– Local variable of
reference typewill point
at heap memory.
• Heap
– Objects.
– Primitive fields of an
object.

3. Mark and Sweep Approach
• Simplest. Garbage collector algorithm.
• Recover and reuse.Heap memoryno longer in use.
• Algorithm.
– Stop-the-world.Non-deterministicpauses stop allthe
running threads.
– Start.From live objectswalks through the tree of
references.
– Mark.As liveany object on the route.
– Sweep.Everything left is garbage and can be collected.

4. Generational Garbage Collector
• Mark-and-sweep. Improved.
• Weak generational hypothesis [2].
– Mostobjectsbecome unreachable
quickly.
– Fewreferencesfrom older to young Eden
Young Generation
objects exist.
• Areas of memory. Survivor
– Eden.Most new objects (very large From To
objects directly to old generation).
Generation
– Survivor.Survived, one holds
Old
Tenured
object, the other empty.
– Tenured.Promoted longer-lived
objects. PermGen
– PermGen.Not strictly in heap, internal
structures (i.e. class definitions).

5. Escape Analysis
• Recent change.Java 6u23.
• Local variables.Onlyused
inside the method.
– No passedinto other
methods.
– No returned.
• No heap.Object created on
the method stack frame
– Reduce objects of young 10
collections.
– Memory used freed when
method returns.

6. Concurrent Mark-Sweep in Action
• Two short pauses.Per GC cycle, initial mark and
remark.
Marking/Pre-cleaning Sweeping
• Initial Mark.Identifies set of objectsimmediately
reachable outside old generation.
• Concurrent marking phase.Marks all live
objectstransitively reachable from this set.
• Object graph can change.Not alllive objects are
guaranteed to be marked.
• Pre-cleaning.Revisitingobjects modified concurrently
with the marking phase.
Initial Mark Remark
• Second Pause (Remark).Revisits objects modified
during concurrent marking phase. Running application thread
• Concurrent sweep phase.Deallocates garbage Running GC thread
objects without relocatingthe live ones.

7. CMS Pros/Cons
• Advantages.
– Two pauses. No one single pause.
– Concurrency. App and GC run in parallel.
• Disadvantages. Start sweeping
– Extra overhead.
– Freelists. Free space not contiguous.
– Large Java heap req.Marking cycle lasts
morethan stop-the-world, spacereclaimed at End sweeping
the end.
– App runs concurrently.Old
generationpotentially increases during
marking phase.
– Floating garbage. Not guaranteed all
garbage objects.
– Fragmentation issue.Lack of
compaction, possible not efficient use of free
space [3].

8. Garbage-First GC (G1) in Action
• Features.
Parallel, concurrent, incrementally
compacting low-pause.
• Heap layout.Split into regions.
• Region.Equal-sized chunks.
• Pause goal.How long app can pause for
GC while running (20 ms every 5 min).
• Pause.Objects evacuatedfrom one or Marking
more regions to a single region.
• Statistics.Average a region takes to
collect.
• G1?. Knows mostly empty regions.
– Collects.Inthis regions first.
– Concentrates.Collect on areas likely
GC GC GC
to be full of garbage.
Running application thread
Running GC thread

9. G1 Pros/Cons
• Advantages.
– High performance.
– Pause time goals. Prevent interruption proportional to heap or live-data size.
– Compact and free up memory.Continuously work to reduce fragmentation.
– Concurrent global marking phase. Determine liveness of the objects.
– Pause prediction model.
• Meet user-defined pause time target with high probability.
• Selects number of regions to collect based on the time target.
• Disadvantages.
– Target. Multi-processor machines with large memories.

10. Comparison
• G1. • CMS.
– Compacting collector. – No compaction.
– Avoid free list, rely on regions. – No control.
– More predictable pauses. • ParallelOld.
– User can specify pause targets. – Whole-heap compaction, long
• Switch to G1. pauses.
– More than 50% heap occupied – No control.
with live data.
– Allocation and promotion rate
varies significantly.
– Undesired long collection and
compaction pauses (0.5s to 1s)

11. References
[1] Lindholm, Tim, and Frank Yellin. Java Virtual Machine Specification, Second Edition.
Addison-Wesley, Reading, MA, 1999.
[2] Charlie Hunt, Binu John. Java Performance, First Edition. Pearson, 2011.
[3] Jones, Richard, and Rafael Lins. Garbage Collection. John Wiley &
Sons, Ltd.,WestSussex, PO19 IUD, England, 1996.
[4] Memory Management in the Java HotSpot Virtual Machine. Sun
Mircrosystem, April 2006.
[5] B.J. Evans, M. Verburg. Well-Grounded Java Developer. Manning, 2012.

12. Questions?